Japanese patent publication No. 2000-263556 discloses a process of making a mold for a micro optical lens. The mold is fabricated by steps of preparing a semiconductor substrate, providing a dielectric mask on a top surface of the substrate, forming one or more openings in the mask, placing the substrate in an electrolytic solution, and anodizing a portion in the top surface not covered by the mask to convert the portion into a porous zone. Thereafter, the porous zone is removed to leave a rounded convex in the top surface of the substrate. An ultraviolet curable resin is placed in the convex and is cured therein to obtain a convex lens. Although the prior art discloses the formation of the porous zones, the process relies upon the mask with the opening so that the porous zone develops isotropically from the center of each opening. Accordingly, the resulting rounded convex is limited to have substantially a uniform radius of curvature. With this limitation, the process cannot be not adapted to fabricate optical lens having non-uniform radius of curvature or sophisticated surface profile.
Further, it is found that the rounded convex revealed by removal of the porous zone suffers from minute projections due to the fact that the porous zone leaves a relatively irregular interface with the semiconductor substrate. This irregularity may detract from lens transmissivity and is desired to be smoothed out when fabricating the lens of good transmissivity.